Branching units are units that connect three cables (e.g. three submarine cables). In general terms, a branching unit may provide a connection between a first location and second location, at the same time as connecting the first and second locations to a third location via a branch connection. Traffic between the first location and the second location is typically carried by a first range of wavelengths, and traffic to or from the third location (from either the first or second location) is carried by a second (different) range of wavelengths.
In general, it is desirable for branching units to be highly reliable, because recovering a branching unit for maintenance is made complicated and difficult by the three cables (and generally requires a complex marine operation). Complexity in a branching unit should therefore be kept to a minimum. It is also desirable that branching units provide for flexibility in spectral allocation (e.g. the allocation of channels in a wavelength division multiplexed system) between each of the first, second and third locations, so that traffic through the branching unit can be more optimally allocated spectrum/channels. For simplicity, the discussion of branching units herein will concentrate on a single fiber and direction of transmission for simplicity, but it will be appreciated that similar principles are applicable to branching units with a plurality of fibers, and for bi-directional communication (whether by separate fibers or in a single fiber).
In some branching units, which may be referred to as “fiber-drop” branching units, one or more fiber pairs are diverted to provide connectivity between the first and third locations. With this approach, traffic between the first and second locations must be routed through the third location, which causes a number of potential problems. A break in the cable between the branching unit and the third location can disrupt the flow of traffic between the first and second locations (or other branch points after the third location).
An alternative form of branching unit, that addresses some of these problems, may be referred to as an optical add drop multiplexing (OADM) branching unit. With this approach, traffic between the first location and the second location is again carried by a first range of wavelengths, and traffic to or from the third location (from either the first or second location) is carried by a second (different) range of wavelengths.
However, in an OADM branching unit, wavelength selective filters are used to divert (drop) only the second range of wavelengths to the third location from the first location, and to recombine (add) the second range of wavelengths from the third location to the second location. This addresses some of the problems with a fiber-drop branching unit, but can bring problems of its own.
Where fixed wavelength filters are used (which is presently typical to avoid concerns over reliability, complexity, and power consumption required for reconfigurable arrangements) the operator is committed to a fixed wavelength allocation for the branch traffic to or from the third location. Given the difficulties in forecasting traffic, there have been efforts to provide greater flexibility without too much complexity (complexity tending to have the drawback of negatively impacting both reliability and cost). One approach has been to site the wavelength selective filters in a box between the branching unit and the third location. This means that filters can be replaced more readily without interfering with the branching unit, which may require a difficult marine operation. Although recovering such a box is also disruptive, it is a simpler operation than recovering a branching unit (where, by definition, at least three cables meet).
Another way to improve flexibility is to use a “broadcast” branching unit. In this arrangement, an optical coupler within the branching unit routes all the wavelengths from the first location to both the second and third locations. A second coupler adds wavelengths from the third location to traffic destined for the second location. This adds flexibility, but the wavelengths added from the third location must be different from those present in the transmissions from the first location. Further branches each require an allocation of wavelengths that are not used in transmission from the first location or from any of the preceding branches. When several branches are used, this approach may become quite inefficient in allocating wavelengths.